Introduction and Core Concepts

This module covers the foundational knowledge you need before writing your first Dockerfile. By the end, you will understand what Docker is, how it evolved, who governs it, and how to evaluate whether it fits your use case.

Why Start Here?

Docker is more than a CLI tool. Its architecture, licensing model, and governance all influence how you plan infrastructure. Understanding the full picture early prevents costly assumptions later.

How Docker Works at a High Level

Docker sits between your application code and the host operating system. It provides a standard packaging format (images) and a lightweight runtime (containers) so that your app behaves the same on every machine.

flowchart TB
    subgraph Client ["Docker Client (CLI)"]
        CMD["docker build / pull / run"]
    end

    subgraph Host ["Docker Host (Daemon)"]
        DAEMON[dockerd]
        IMG[Images]
        CONT[Containers]
        VOL[Volumes]
        NET[Networks]

        DAEMON --> IMG
        DAEMON --> CONT
        DAEMON --> VOL
        DAEMON --> NET
    end

    subgraph Registry ["Container Registry"]
        HUB[("Docker Hub")]
        PVT[("Private Registry")]
    end

    Client -->|REST API / Socket| DAEMON
    DAEMON <-->|Pull / Push| Registry

You interact with Docker through the CLI (client). The CLI sends instructions to the Docker daemon (dockerd) running on the host. The daemon manages images, containers, volumes, and networks — and can pull or push images from a registry like Docker Hub.

Core Terminology

Term	What It Is	Analogy
Image	A read-only template containing your application and its dependencies	A recipe — you follow it to make the same dish every time
Container	A running instance of an image, isolated from the host and other containers	A dish made from the recipe — disposable, reproducible
Volume	Persistent storage that survives container deletion	An external hard drive
Network	A virtual connection between containers	An ethernet switch connecting devices
Dockerfile	A text file with instructions for building an image	The recipe card itself
Compose	A tool for defining and running multi-container applications from a single file	A conductor orchestrating multiple musicians

Containers vs Virtual Machines

If you have used VMs before, you will notice containers feel much lighter. Here is why.

A virtual machine runs a full guest operating system on top of a hypervisor. A container shares the host kernel and only packages the application layer.

flowchart BT
    subgraph VM ["Virtual Machine"]
        APP1[App] --> LIB1[Libs] --> OS[Guest OS] --> HYP[Hypervisor] --> HW1[Infrastructure]
    end

    subgraph CONT ["Docker Container"]
        APP2[App] --> LIB2[Libs] --> ENG[Docker Engine] --> HW2["Host OS + Infrastructure"]
    end

Feature	Virtual Machines	Containers
Isolation	Hardware-level (heavy)	OS-level (lightweight)
Startup Time	Minutes	Seconds
Size	Gigabytes	Megabytes
Performance	Near-native	Native

Both tools have their place. Use containers for application packaging and microservices. Use VMs when you need full OS isolation or must run a different kernel.

Is Docker Free?

Yes — the core technology is free and open source.

Component	License	Cost
Docker Engine (Linux)	Apache 2.0	Free
Docker CLI	Apache 2.0	Free
Docker Desktop (Mac/Windows)	Proprietary	Free for personal, education, and small business use. Paid subscription for larger organizations.

Production Note

On Linux servers, you use Docker Engine directly. It is completely free and unaffected by the Docker Desktop licensing changes.

What You Will Learn in This Module

Lesson	What It Covers
What Is Docker and Why It Matters	Core concepts, the image/container model, and a hands-on walkthrough
History of Docker	How Docker evolved from a PaaS side project to an industry standard
Licensing and Governance	What is free vs paid, who controls the ecosystem, and what your team needs to track
Docker Popularity and Industry Adoption	Market adoption, use cases, and where Docker fits in the modern stack